2 of every species on one boat would be a logistical nightmare and would take decades to scour the earth and capture, feed, cultivate the millions of species on earth. Even today, with all the resources at hand, it would be impossible to achieve without many of them dying.
Not two of each animal, two of each "unclean" animal, and seven of each "clean" animal. (This in light of the fact that Moses had yet to be born to tell anyone what was clean and what was unclean.)
Now to logistics, the Ark was 450 feet long, 75 feet wide and 45 feet high. It thus has a volume of 1,518,750 cubic feet. It therefore had a displacement of 1,518,750 divided by 100 = 15,187.5 tons. (tonnage calculations)
15,187.5 tons of space is available on the Ark before it sinks. Let's first consider the weight of the creatures aboard themselves. While we can argue about what "Kind" means, we'll use the Creationist number of 16,000 animals.
16,000 x 763 = 12,208,000 lbs 12,208,000 pounds divided by 2000 (pounds per ton) = 6,104 tons.
15,187.5 - 6,140 = 9,047.5 tons left for food to feed these beasts.
Lets see what we have now. A tonnage capacity of 15,187.5
(Less) Biomass weight = 6,140
(Less) Food stores weight = 75,498.5 (1/30th of each animals body weight per day.)
Equals -66,451 Tons.
Well, The Ark is sunk, and we haven't even addressed fresh water yet. Likewise, we haven't considered that the Earth was destroyed of all life, and these creatures - once offloaded, would not have anything to eat until the Earth repopulated itself with plants, thus Noah would have needed even more food with him.